This invention relates to an optical switching device for switching an optical signal in a communications network, and in particular to an optical switching device using an optical loop.
In evolving optical communication networks, the need often arises to switch an optical signal from one path to another, be it along a waveguide or in free space. For example, in a network, which consists of a number of communication nodes connected sequentially to form a ring, it may be required to temporarily remove one or more nodes from the network. Therefore the optical waveguides, for example in the form of optical fibers interconnecting the network must be able to xe2x80x9cswitchxe2x80x9d the node(s) from an active (transmit/receive) state to a passive (bypass) state. Many arrangements exist in the art for providing this type of switching.
An exemplary optical fiber switch that utilizes a moving mirror to perform the switching function is disclosed by Levinson in U.S. Pat. No. 4,580,873 issued Apr. 8, 1986 which is incorporated herein by reference. Another switch of the former design is disclosed by Benzoni in U.S. Pat. 5,042,889 and assigned to ATandT Bell Laboratories and issued in 1991. Benzoni""s invention relates to an activation mechanism for moving a reflective element in or out of the path of a beam of light, to allow light to be either transmitted in a first position or reflected in a second position. Both inventions rely on moving mechanical parts limiting the speed of switching and causing reliability problems. Furthermore, mechanical switches are too complex and too expensive to manufacture for use in large optical communication networks.
To overcome the inherent problems of mechanical switching optical switches relying on interferometric techniques have been developed. One such example is a Mach-Zehnder interferometer as disclosed by M. J. LaGrasse et al. in xe2x80x9cUltrafast Switching with a Single Fiber Interferometerxe2x80x9d, Optics Letters, Mar. 15, 1989, Vol. 14, No. 6, pp. 311-313. The Mach-Zehnder interferometer accepts a signal, splits it into two parts that are sent over two distinct paths, and recombines the signal of the two paths. Switching is achieved by including a phase shifting element in one of the two paths. Unfortunately, the two signal parts travel different paths, which do not occupy a same space. As a result, the constructive and destructive combination of the two signal parts at the output is subject to different temperatures, pressures, electric fields and other extraneous factors and cannot be reliably controlled.
Exemplary optical switches that utilize a Sagnac interferometer to perform the switching function are disclosed by Giabriel et al. in U.S. Pat. No. 5,144,375 issued Sep. 1, 1992, Blow et al. in U.S. Pat. No. 5,307,428 issued Apr. 26, 1994, and Blow et al. in U.S. Pat. No. 5,473,712 issued Dec. 5, 1995, which are incorporated herein by reference. Switching is activated by sending an optical pump pulse concurrently with the optical signal affecting the polarization of two signal portions travelling through the Sagnac interferometer. Use of optical pump pulses for reliably controlling optical switches is difficult to apply in large optical communication network where a plurality of switches is cascaded. Switching by sending an optical pump pulse is, furthermore, difficult to implement in wavelength division optical communication networks comprising a plurality of wavelength channels transmitted in one fiber.
It is an object of the invention to provide an optical switch for reliably switching an optical signal in a large optical communications network that is controlled at high speed using an electric signal.
It is further an object of the invention to provide an optical switch that is easily implemented in a wavelength division optical communication network.
In accordance with the invention there is provided, an optical switching device for controlling an optical signal. The switching device comprises: two ports disposed for optical communication therebetween for launching or receiving light;
a polarization rotating element interposed between the two ports for rotating the polarization of light incident thereon; and, a filtering element disposed to bypass the polarization rotating element or direct a signal propagating between the two ports through the polarization rotating element in dependence upon a property of the light.
In accordance with the invention there is provided, an optical switching device for controlling an optical signal. The switching device comprises: an optical loop, the loop having two ports for launching or receiving light; a switching element interposed within the loop for selectively routing light, the switching element comprising:
a polarization rotating element for rotating the polarization of light incident thereon; and, a filtering element disposed to bypass the polarization rotating element or direct a signal propagating within the loop through the polarization rotating element in dependence upon a property of the light.
In accordance with the invention there is further provided, a method for switching an optical signal. The method comprises the steps of: receiving an optical input signal from one of two ports; rotating the polarization of the optical input signal using a polarization rotating element interposed between the two ports; and, using a filtering element for bypassing the polarization rotating element or for directing a signal propagating between the two ports through the polarization rotating element in dependence upon a property of the light.
In accordance with another aspect of the invention there is provided, a transmultiplexer for interchanging optical signals between two optical fibers. The transmultiplexer comprises:
an optical loop, the loop having two ports wherein each port is connected to one of the two optical fibers for launching or receiving light; at least a switching element interposed within the loop for selectively routing light, the switching element comprising:
a polarization rotating element for rotating the polarization of light incident thereon; and, a filtering element disposed to bypass the polarization rotating element or direct a signal propagating within the loop through the polarization rotating element in dependence upon a property of the light.